CH stretching vibrations of jet-cooled benzene-acetylene and several aromatics-acetylene clusters were observed in order to characterize the intermolecular interaction, so-called activated CH/pi interaction between an acidic CH group and a-electrons. The infrared-ultraviolet double resonance spectroscopic techniques were used for measuring their vibrational spectra. The antisymmetric CH stretching vibration of the acetylene moiety exhibits a remarkable low-frequency shift upon the cluster formation with the aromatic molecules, indicating such a cluster structure that the acetylenic CH group is bound to pi-electrons of the aromatic ring. The low-frequency shifts observed for the clusters show a positive correlation with the a-electron density of the ring, as expected for an ordinary pi-hydrogen bond. On the other hand, the polarizability of the proton-accepting molecules showed a weakly negative correlation with the CH frequency shifts. These results show that the intermolecular interaction between the activated CH group and,pi-electrons is characterized as a a-hydrogen bond rather than as a van der Waals interaction.